Commercial recovery of valuable minerals, such as potash, from brine solutions is carried out in many parts of the world by establishing large-scale evaporating ponds. For example, the production of minerals from the Dead Sea is based on evaporating ponds whose total area exceeds 130 Km.sup.2. Solar radiation absorbed by these ponds supplies latent heat of evaporation, thus allowing successive ponds to become progressively more concentrated, until the desired minerals precipitate and can be harvested.
Because the vapor pressure of brine decreases with increasing concentration, and is small in comparison with fresh water at the same temperature, evaporation is retarded as the brine concentration increases. Considering the Dead Sea as an example, the annual evaportion of water from Dead Sea brine varies between 1.4 and 0.7 m. per year, depending on the concentration of the brine, while about 2.2 m. per year is evaporated from fresh-water ponds in the same region.
Absorption of solar radiation by an evaporating pond raises the temperature of the brine therein until evaporation takes place and a steady-state condition is established. The elevated temperature of the pond increases the sensible and long-wave radiation flux from the ponds to a level that exceeds the latent heat flux that produces evaporation and concentration of brine. Thus, only a fraction of the solar energy input to an evaporating pond is converted into useful latent heat flux. Commercial production of useful minerals by evaporating ponds is thus inefficient in its use of solar energy, and is limited by the area availble for the evaporating ponds.
A need exists, therefore, for a technique that is more effective than conventional evaporating ponds for evaporating water from brine in order to increase production of useful minerals without increasing the area dedicated to their production.
As noted in U.S. Pat. No. 4,351,849 (the disclosure of which is hereby incorporated by reference), food, cosmetic and other industries are also heavily engaged in processes that evaporate liquids from solutions to concentrate the latter, or to precipitate solids from the solutions. As described in the '849 patent, the drying process involves a starting solution containing solids dissolved or suspended therein, and a gaseous drying atmosphere whose temperature, pressure, and vapor content establish a condition in which the atmosphere is capable of absorbing a substantial quantity of liquid from the starting solution. Normally, the gaseous drying atmosphere is air that is heated substantially above room temperature; and the drying process is carried out by dispersing the liquid to be dried in the drying atmosphere as droplets small enough so that the liquid component of the droplets will be evaporated readily. When the starting solution is a food product such as a bakery mix product, vegetable or fruit juice, or milk or a diary product, the suspension liquid, ordinarily, is water. Where the starting solution is a chemical, the suspension liquid may be a non-polar organic solvent such as a hydrocarbon liquid or other petroleum derivative, aldehyde, ketone or other solvent known in the art.
Solids contained in the starting liquid may comprise dissolved solids present as a true solution as well as colloidal and non-colloidal suspensions of insoluble material. Thus, the term "solid" is used herein to include suspended material which sometimes exist as a liquid (e.g., fat, in the cse of a dairy product). Such material may be chemically pure of may be a mixture.
As explained in the '849 patent, the temperature of the drying atmosphere must be high enought to enable the drying atmosphere to hold a substantial quantity of liquid or solvent being evaporated. But, the temperature should not be so high that it will cause chemical degradation or decomposition of the material being dried. Drying heat sensitive products, particularly food products, using known techniques exemplified in the '849 patent, often produces products whose appearance, taste, and other properties are affected adversely.
A need exists, therefore, for a drying technique that does not require heating of the gaseous atmosphere to temperatures in excess of those that adversely affect the quality of the resultant dried material, yet effectively and quickly removes liquid from the starting solution.